TMC2209 - UART only working with two or less drivers connected

[botboe]

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Hi everyone,

I'm using the TMCStepper lib indirectly via the FluidNC-firmware for ESP32-based CNC-routers and have a strange behavior when using the uart of the TMCs. I use a MKS TinyBee controllerboard with three MKS TMC2209 v2.1 drivers with manually exposed UART-Pin connected via star-shaped cable to USART2 of the ESP32 like in this picture: Each driver has its own address configured and can be queried on its own by the firmware correctly. If I connect a second driver it still works for both of them. But when I add a third driver, the connection fails for all drivers. It doesn't matter which drivers are connected, but it seems to be depending on the number of them.

Do you have any idea what the issue might be?